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Homework Help: Kinematics - Acceleration in Two Dimensions. Stuck.

  1. Jan 29, 2009 #1
    Hi, I am also having trouble with the hockey puck question.

    A hockey puck rebounds from a board as shown in my diagram. The puck is in contact with the board for 2.5 ms. Determine avg acceleration of the puck over the interval.

    Vi = 26 m/s Vf = 21 m/s

    I tried the cosine law but I keep getting 44 m/s and not 18. I don't understand how you guys got 18 m/s. I've plugged it in at least 100 times as
    v_t = \sqrt{v_1^2 + v_2^2 - 2(v_1)(v_2)cos136}
    = \sqrt{26^2 + 21^2 - 2(26)(21)cos136}
    = [/itex]

    Because that wasn't working I then tried Vector Components and I can't get that to work either. I did:
    V_x = V_B sin \theta + (-V_A cos \beta )
    = 21 sin(22) - 26 cos(22)


    V_y = V_B cos \theta + (-V_A sin \beta )
    = 21 (cos22) + 26(sin22)
    = 29

    I then tried to figure out
    \Delta V ^2= \Delta V_x ^2 + \Delta V_y^2
    = sqrt{16^2 + 29^2}
    = 33

    Using that I tried to get the average acceleration by:

    A_av = \Delta V / \Delta T

    A_av = 33 / 2.5x10^-3
    A_av = 13.2x10^3
    and to find the angle I tried to do :

    \phi = tan^-1 = 16/29
    \phi = 29degrees

    However, the answer in my book says that the average acceleration is [itex] 7.3x10^3 [7.5degrees North of West][/itex]

    Any help would be amazingly appreciated. Thanks.

    Attached Files:

  2. jcsd
  3. Jan 29, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You have a mix of sine and cosine. Only one is correct.
    Again, a mix of sine and cosine.

    Redo this.
  4. Jan 29, 2009 #3
    I don't know how to redo it. In my text book it used them both together in the y and x component vector subtraction. I took the equations right out of my text, Nelson Physics 12.
  5. Jan 29, 2009 #4

    Doc Al

    User Avatar

    Staff: Mentor

    I'm not sure what equations you are talking about.

    Do this: What's the x-component of Vi? The x-component of Vf?
  6. Feb 8, 2009 #5
    I have the same book and I am stuck on the example on right before the quesion box you asked about. can you please explain the lawn mower example in pg 28. I AM VERY CONFUSED
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